1. Field of the Invention
The present invention relates generally to the projection of images on a screen and more specifically to an optical reflector capable of achieving high utilization efficiency.
2. Description of the Related Art
A prior art image projector has a point-source lamp 100 located at the focal point of a parabolic reflector 101 as shown in FIG. 1A or at one of the two focal points of a spheroidal reflector 102 as shown in FIG. 1B. Rays from a point source 100 are reflected off in different ways as illustrated depending on the concave shape of the reflector. In the case of parabolic reflector 101, reflected rays run parallel along the optical axis of the reflector, whereas the rays reflected off the spheroidal reflector 102 converge to the other focal point of the reflector 102. Depending on the specific needs of projection systems, one of these reflectors is employed.
The reflected light is modulated imagewise by passing it through a liquid crystal display (LCD) panel on which a video image is produced and the modulated light is projected onto a screen. To irradiate the LCD panel with uniform light intensities, a fly-eye-lens system (known as an integrator) is used to uniformly distribute the flux densities. In addition, since the liquid crystal panel utilizes light of only one polarization component, it is the usual practice to employ a light enhancer to increase the amount of useful light. This enhancer is composed of an array of light shields where rays from the fly-eye-lens system are masked and the unmasked potions are allowed to enter an array of beam splitters where they are split into first and second polarization components, the first being the useful component and the second being the otherwise useless component. The light enhancer is further provided with an array of reflectors and associated phase shifters. On each of the reflectors the second component of light from the corresponding beam splitter is reflected off to the associated phase shifter, where it is rotated and aligned to the polarization plane of the first component and emitted in parallel therewith.
However, aberrations of light occur on the surface of the light enhancer due to finite dimensions of the point-source lamp. Such aberrations are particularly severe if the projector uses a light reflector of the spheroidal type due to its unparallel light rays to which the light enhancer is exposed. More specifically, the fly-eye-lens system is composed of two parallel plates, each having a matrix array of cells, or microscopic lenses. They are spaced at such a distance apart so that rays leaving one cell of the first plate are focused onto a corresponding cell of the second plate. Because of the unparallel paths of light from the spheroidal reflector, rays incident on the first fly eye lens plate are caused to progressively increase their aberration as they pass through the fly-eye-lens system. Further, due to the aberrations caused by the finite dimensions of the point-source light, rays incident on each cell of the second fly eye lens plate are formed into an elliptical shape, and the size of such shapes on the outer areas of the second fly-eye-lens plate is greater than the size of those in the inner areas. Therefore, uniformity of densities is lost, known as "flares", at the entry surface of the light enhancer, and hence a large amount of luminous flux is shielded off by the light enhancer, resulting in a substantial loss of useful light for imagewise modulation.
On the other hand, the rays from the light reflector of paraboroidal type are less severe in aberration at the entry surface of the light enhancer than the rays from the spheroidal reflector, and hence higher light utilization efficiency is possible. However, if the aperture of the parabolic reflector is limited, it is impossible to increase its dimensions along the optical axis to increase the amount of light for projection. Thus, the parabolic reflector has a lower reflectivity than the spherical reflector. One approach is to employ a part-spherical auxiliary reflector at the aperture of a parabolic main reflector for reflecting portions of outwardly emitted light back to the interior of the main reflector, as disclosed in Japanese Laid-Open Patent Specification 8-262437. However, the reflector and hence the projection system as a whole becomes undesirably bulky.
Although the longitudinal dimensions of the parabolic reflector can be increased for a given size of aperture if the reflector may be designed with a short focal point, there is interference between the point-source lamp and the interior surface of the reflector on the one hand, and there is, on the other, an increase in the amount of rays lost by the throughhole provided in the reflector for the electrodes of the light source.